Human locomotion studies made at the University of California, Berkeley, in 1950, provided the basic data for theoretical calculations of mechanisms which would be used in an above-knee prosthesis to approximate the walking gait of a normal person. The prior art mechanical friction knee mechanisms were shown to have a fundamental deficiency in that the devices provided the same amount of frictional resistance to movement at slow cadences as to movement at fast cadences. As a result, adjustment for absorbing the desired energy during swing phase was only optimum at one cadence. At faster cadences, braking was insufficient, resulting in excessive heel rise and hyperflexion. At slow cadences, the braking resulted in inadequate flexion and extension. It was found that the use of hydraulic damping in the knee mechanism could be used to achieve nearly normal leg action over a wide cadence range. This is due to the characteristics of hydraulic flow through ports or orifices in which the resistance to flow increases with increasing cadence at a rate almost exactly balancing the energy absorbing requirements of the prosthesis. This permits a design that provides for the correct heel rise and extension in swing phase, independent of cadence.
In hydraulic damping devices, the device usually includes a hydraulic cylinder and piston unit defining an annular chamber between the inner wall of the cylinder and the piston for storing a hydraulic medium. The piston is provided with an annular shoulder which divides the storing chamber into two partial chambers, each being connected via damping passages so that the hydraulic medium may pass from one partial chamber into the other, and vice versa. Usually, the partial chambers are arranged in axial relation to one another.
Also known in the prior art is to provide two cylinder spaces of variable volume, whereby the piston, during its movement, reduces the volume of the first partial chamber and the hydraulic fluid flows through connecting conduits into the correspondingly enlarged second partial chamber. For example, during the flexion cycle of the leg movement, the piston would move downward and the fluid flows through the connecting conduits to the upper chamber. As the piston moves downward, the connecting conduits are covered by the piston, causing pressure to build up below the piston and causing programmed deceleration of the prosthesis. A series of adjustable valves are used to control the peak value of the deceleration and the amount of energy absorbed in limiting heel rise. Thus, by adjusting the valves, the amount of resistance in the prosthetic leg can be increased or decreased. The same action occurs during the leg extension cycle as well, except that in the leg extension cycle, the piston usually moves upward through the cylinder. As the piston moves upwards, the connecting conduits are covered and pressure builds above the piston to cause programmed deceleration of the prosthesis in a natural manner. Again, a series of adjustable valves are used to control peak deceleration and, thus, the amount of energy absorbed in stopping extension travel.
The main disadvantages of the prior art valve system hydraulic control units is that some are rather large and bulky, so that they require a relatively large installation space; and most are difficult to manufacture, since they require many parts.
Other prior art hydraulic control units include U.S. Pat. Nos. 5,405,409 and 5,443,521 to Knoth, U.S. Pat. No. 4,595,179 to Glabiszewski, and U.S. Pat. No. 5,376,135 to Aulie. The Knoth patents disclose a hydraulic control device which uses fluid control ports, channels and adjustable gaps defined by an axially adjustable sleeve and a control bushing for damping the movement of the piston rod. The piston rod encloses a preferably sealed gas pressurized flexible bladder which forms an oil accumulator during inward movement of the piston rod into a displacement chamber and also produces variable forces for moving the piston rod outwardly to its extended position. The Glabiszewski patent discloses a hydraulic damping unit for use in artificial joints where a hollow throttling piston is guided in a main piston and is formed with an annular recess which communicates with respective variable volume chambers via passages passing through the jacket of the main piston at both sides of the pistons shoulder. In the range of both passages, the bottom of the recess of the throttling piston is connected to the outer surfaces of the latter via a sloping annular surface which depending on the relative position of the throttling piston to the main piston, adjusts the clearance of the passages to control the resistance to flow of the hydraulic liquid. The Aulie patent discloses an adjustable hydraulic damper involving a piston member within a hydraulic cylinder where the hydraulic cylinder wall is adjustably deformed to vary the annular space between a piston head and the inside cylinder wall through which hydraulic fluid must pass. The wall of the cylinder is adjustably deformed through an annular ramp formed circumferentially about the outside of the cylinder and having an angled bearing surface.